1. Field of the Invention
The present invention relates to an effective technique applied to an arrangement of an optical path cross-connect system used to construct a large-scaled optical network in correspondence with an increase in a total number of wavelengths.
2. Description of the Related Art
Very recently, while large amounts of information are communicated in high speeds, very broad band networks as well as very wide range transfer systems with large capacities are required. As one of means capable of realizing such needs, a WDM-technique based optical network is desirably constructed. A core device required when such an optical network is constructed corresponds to an optical path cross-connect (optical XC).
FIG. 1 illustratively shows a typical structural example of an optical XC system and an optical network. As indicated in this drawing, the optical path cross-connect (XC) device is such a device which contains a plurality of input/output optical transmission lines, and routes wavelength-division multiplexed optical signals which are entered from the input optical transmission line into a desirable output optical transmission line wavelength-by-wavelength. When a long distance transmission line is constructed, an optical amplifier is inserted to the optical transmission lines between the optical XC device, and the optical XC device is connected to another communication device (for example, electric cross connect: electric XC) through the intra-office optical transmission lines. Then, these devices are controlled by an operation system for managing the entire network.
On the other hand, a total number of wavelengths is rapidly increased in an optical network in connection with an increase of traffic capacity. However, when a total number of wavelengths is increased, a system scale required for the optical XC device is increased, which may cause a practical difficulty.
In the optical XC system, there are two different types, i.e., a wavelength fixing type XC system in which a wavelength is not converted within a node; and a wavelength converting type XC system in which a wavelength is converted within a node, if required. FIG. 2(a) and FIG. 2(b) represent general structures of the respective wavelength fixing/converting type XC systems with employment of optical switches. The wavelength fixing type XC system indicated in FIG. 2(b) is constituted by a wavelength-division demultiplexer, an optical switch unit, and a wavelength-division multiplexer. Since the optical switch is controlled in this wavelength fixing type XC system, an input optical signal is routed to a desirable output transmission line while keeping the wavelength thereof unconverted.
On the other hand, the wavelength converting type XC system indicated in FIG. 2(a) requires a wavelength converter used to convert the wavelength (note that output wavelength is fixed), as compared with the above-explained wavelength fixing type XC system, and also controls the optical switch in order to convert the wavelength of this input optical signal into a desirable wavelength of a desirable output transmission line.
It should be understood that as an example of the wavelength converter, there are provided two different converting systems, namely a wavelength of an input optical signal is directly converted into a desirable wavelength while maintaining the optical signal form by utilizing an optical semiconductor element, and a wavelength of an input optical signal is converted into a desirable wavelength by using both an optical/electric converter and an electric/optical converter. Also, as an example of the optical switch, there are provided a dielectric element such as LiNbO3, an optical semiconductor element such as InP and GaAs, a sillica-based waveguide switch realized by utilizing the thermo-optic effect, and a mechanical optical switch realized by utilizing a stepper motor and a prism. Furthermore, as an example of the wavelength-division demultiplexer and the wavelength-division multiplexer, such elements may be employed which use an array waveguide grating and a dielectric multilayer film.
FIG. 3 is a conceptional view indicating an optical path network in such an optical network established when the conventional optical XC device indicated in FIG. 2 is employed.
As shown in FIG. 3(a), in the wavelength converting type optical path network, a wavelength is allocated in a link-by-link basis between a sender and a receiver node with respect to a single optical path. In other words, a wavelength is converted with respect to each of repeating optical XC device, if required.
On the other hand, as shown in FIG. 3(b), in the wavelength fixing type optical path network, a single wavelength is allocated between a sender and a receiver node with respect to a single optical path. In other words, the wavelength is not converted within a repeating optical XC device.
In this case, the below-mentioned problems occur, comparing the wavelength fixing type optical path network with the wavelength converting type optical path network.
That is, in the wavelength fixing type optical path network, when the optical signal having the same wavelength as that of the wavelength-multiplexed optical signal which is entered from the input optical transmission line is routed to the same output optical transmission line, blocking will occur.
On the other hand, since the wavelengths can be converted in the wavelength converting type optical path network if necessary, even when the optical signals having the same wavelengths are routed to the same output transmission line, the optical signal can be routed without any occurrence of such blocking. However, a large-scaled optical switch is required, as compared with that of the wavelength fixing type optical path network. Furthermore, in the wavelength converting type optical path network, when a total number of wavelengths is increased, the scale of the optical switch must be enlarged (namely, this optical switch must be replaced by another large-scaled optical switch). As a consequence, the wavelength converting type optical path network is not superior in the expanding characteristic with respect to a total number of wavelengths. To the contrary, in the wavelength fixing type optical path network, a total number of optical switches may be increased, depending upon an increase of wavelength number.
In summary, there are the following problems that the transfer characteristic (blocking characteristic) is deteriorated in the wavelength fixing type optical path network, whereas the wavelength converting type optical path network has no expanding characteristic with respect to increasing of the wavelength number, and the scale of the entire device is enlarged.
The present invention has been made in view of such problems, and therefore, has an object to realize an optical path cross connect technique with a high expanding characteristic with respect to an increase in a total number of wavelengths, while maintaining a better transfer characteristic.